Fully differential capacitive architecture for MEMS accelerometer

ABSTRACT

A fully differential microelectromechanical system (MEMS) accelerometer configured to measure Z-axis acceleration is disclosed. This may avoid some of the disadvantages in traditional capacitive sensing architectures—for example, less sensitivity, low noise suppression, and low SNR, due to Brownian noise. In one embodiment, the accelerometer comprises three silicon wafers, fabricated with electrodes forming capacitors in a fully differential capacitive architecture. These electrodes may be isolated on a layer of silicon dioxide. In some embodiments, the accelerometer also includes silicon dioxide layers, piezoelectric structures, getter layers, bonding pads, bonding spacers, and force feedback electrodes, which may apply a force to the proof mass region. Fully differential MEMS accelerometers may be used in geophysical surveys, e.g., for seismic sensing or acoustic positioning.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/190,673, filed Feb. 26, 2014, which claims priority to U.S.Provisional Application Nos. 61/785,851, filed Mar. 14, 2013, and61/786,259, filed Mar. 14, 2013. All of the above applications areincorporated by reference herein in their entireties.

BACKGROUND

Microelectromechanical system (MEMS) accelerometers are widely used inmany different application areas such as geophysical surveying,underwater imaging, navigation, medical, automotive, aerospace,military, tremor sensing, consumer electronics, etc. These sensorstypically detect acceleration by measuring the change in position of aproof mass, for example, by a change in the associated capacitance.Traditional capacitive MEMS accelerometers may have poor performance dueto low noise suppression and sensitivity, however.

Measurement noise and range may vary for different applications ofsensors. For example, for a navigation application, a measurement rangeof ±20 g may be desired and 1 μg/√Hz measurement noise for this rangecould be tolerated. As another example, a tremor sensing application maydesire a ±1 g measurement range and a lower noise floor of ˜10-100ng/√Hz. The main type of noise affecting this noise floor is Browniannoise. Brownian noise refers to noise produced by Brownian motion.Brownian motion refers the random movement of particles suspended in aliquid or gas resulting from their bombardment by the fast-moving atomsor molecules in the liquid or gas.

Accelerometers may have many uses in the field of geophysical surveying,particularly marine seismic. For example, in some marine seismicembodiments, a survey vessel may tow one or more streamers in a body ofwater. Seismic sources may be actuated to cause seismic energy to travelthrough the water and into the seafloor. The seismic energy may reflectoff of the various undersea strata and be detected via sensors on thestreamers, and the locations of geophysical formations (e.g.,hydrocarbons) may be inferred from these reflections.

These streamer sensors that are configured to receive the seismic energymay include accelerometers such as those described in this disclosure.(Various other sensors may also be included in some embodiments, such aspressure sensors, electromagnetic sensors, etc.)

Additionally, accelerometers may be used to detect the relativepositions of the streamers (or portions thereof) via acoustic ranging.Acoustic ranging devices typically may include an ultrasonic transmitterand electronic circuitry configured to cause the transceiver to emitpulses of acoustic energy. The travel time of the acoustic energybetween a transmitter and receivers (e.g., accelerometers) disposed at aselected positions on the streamers is related to the distance betweenthe transmitter and the receivers (as well as the acoustic velocity ofthe water), and so the distances may be inferred.

In other marine seismic embodiments, accelerometers according to thisdisclosure may also be used in permanent reservoir monitoring (PRM)applications, for example at a seafloor. Generally, the term“geophysical survey apparatus” may refer to streamers, PRM equipment,and/or sensors that form portions of streamers or PRM equipment.

Accordingly, improvements in accelerometer technology (e.g., allowingbetter performance and/or lower cost) may provide substantial benefitsin the geophysical surveying field, among other fields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating one embodiment of a device;

FIG. 2 is a block diagram illustrating one embodiment of a MEMSaccelerometer;

FIGS. 3A-C illustrate an exemplary process flow for the fabrication of acap substrate;

FIGS. 4A-F illustrate an exemplary process flow for the fabrication of afully differential MEMS accelerometer;

FIGS. 5A-E illustrate an exemplary process flow for the etching ofcavities within a substrate; and

FIGS. 6-7 illustrate methods for the use of accelerometers in ageophysical survey according to this disclosure.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Various units, circuits, or other components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/circuits/components include structure (e.g., circuitry) thatperforms the task or tasks during operation. As such, theunit/circuit/component can be said to be configured to perform the taskeven when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invoke 35U.S.C. §112, sixth paragraph, for that unit/circuit/component.

DETAILED DESCRIPTION

Turning now to FIG. 1, a block diagram illustrating one embodiment of adevice 100 is shown. Device 100 includes upper substrate 110, interiorsubstrate 130, and lower substrate 150. In various embodiments,substrates 110, 130, and 150 contain wafers 110 a, 130 a and 130 f(regions of the wafer on substrate 130), and 150 a respectively. Invarious embodiments, these wafers may be silicon wafers. As used herein,the term “wafer” is used broadly to refer to any material used forfabricating microelectromechanical system (MEMS) devices. As will berecognized by one skilled in the art with the benefit of thisdisclosure, “depositing” material on a substrate may occur according tovarious methods common in the MEMS device field. In some embodiments,this deposition method is performed as described below with reference toFIGS. 3 and 4. As depicted, interior substrate 130 is split into threeportions, proof mass 130 a and anchor regions 130 f. In the illustratedembodiment, these portions are separated by cavities 130 g. Proof mass130 a may also be referred to as a proof mass region. Cavities 130 g maybe etched by various methods recognized by one skilled in the art,including one described below with reference to FIG. 5.

In the embodiment shown, upper substrate 110 is bonded to interiorsubstrate 130, and lower substrate 150 is bonded to interior substrate130. Bonding may occur using any suitable method known in the art. Inone embodiment, bonding between substrates 110 and 130 and between 150and 130 occurs using precision gap control, which is described brieflywith reference to FIG. 4. In one embodiment, cavity 120 between uppersubstrate 110 and interior substrate 130 and cavity 140 between lowersubstrate 150 and interior substrate 130 are vacuum-sealed. Cavities 130g may also be vacuum-sealed. In some embodiments, cavities 120 and 140may be vacuum-sealed in part by bonding substrates 110, 130, and 150together in a vacuum environment. Substrate 130 may have portions etchedaway such that vacuum-sealed cavities 130 g, cavity 120, and cavity 140may be in fluid communication with each other (e.g., they may possess acommon vacuum).

In one embodiment, substrates 110, 130, and 150 are divided into twoparts: the wafers of each substrate (110 a, 130 a and 130 f together,and 150 a respectively), and a set of electrodes (110 b & 110 c, 130 b &130 c, 130 d & 130 e, and 150 b & 150 c). Two sets of electrodes may bedeposited/situated/disposed on interior substrate 130: the first on theupper surface, forming electrodes 130 b and 130 c; and the second on thelower surface, forming electrodes 130 d and 130 e. Said differently, thesets of electrodes on the interior substrate are deposited on the topand bottom of the interior substrate, or on opposite sides of theinterior substrate. (Note that the phrase “opposite sides” of astructure such as a substrate is not limited to the top and bottom of astructure; instead, the phrase may be used to variously refer to theleft and right sides of a structure, or the front and back sides of astructure. Of course, the characterization of different portions of astructure as top, bottom, left, right, front, and back depends on aparticular vantage point.)

In one embodiment, a set of electrodes is deposited on the lower surfaceof upper substrate 110, forming electrodes 110 b and 110 c. A set ofelectrodes is also deposited on the upper surface of lower substrate150, forming electrodes 150 b and 150 c. Both of these sets ofelectrodes on upper substrate 110 and lower substrate 150 may bereferred to as a set of electrodes deposited, situated, or disposed onan opposing surface (i.e., the respective upper and lower surfaces ofinterior substrate 130). In some embodiments, sets of electrodes (110 band 110 c, 130 b and 130 c, 130 d and 130 e, and 150 b and 150 c) may bedeposited as a metallic layer.

As used herein, “opposing” surfaces are those that face each other. Asused herein, the term “deposited” refers to any fabrication technique inwhich a type of material is placed on at least a portion of anunderlying material or layer. The term “layer” is to be construedaccording to its ordinary usage in the art, and may refer to a materialthat covers an entire portion of one or more underlying materials, aswell as discrete regions situated on top of the underlying material(s).Accordingly, a “layer” may be used to refer to the set of electrodesdepicted in FIG. 1, which may result from a continuous deposition ofmaterial that is deposited and then partially etched away. In someembodiments—for example as described below with reference to FIG. 4—acertain layer may fall “below” another layer that was deposited firstbecause the first deposited layer is not continuous. For example, adeposition of a piezoelectric material may be processed such that thelayer contains discrete portions. Accordingly, when another spring layeris deposited, some portions of the spring layer may fall “below” thepiezoelectric layer since it is not continuous. Thus, portions of thespring layer may appear to be at the same vertical level as thepiezoelectric layer. Accordingly, in some instances, the term “layer”refers to the order of deposition, and not necessarily the verticalposition (e.g., height) of materials in reference to one another.

In the embodiment shown, the set of electrodes on the upper substrate110 and the set of electrodes on the upper substrate of the uppersurface of interior substrate 130 are configured to form two capacitors.Electrodes 110 b and 130 b are configured to form one capacitor;electrodes 110 c and 130 c, the other capacitor. Similarly, the set ofelectrodes on lower substrate 150 and the set of electrodes on the lowersurface of interior substrate 130 are configured to form two capacitors.Electrodes 150 b and 130 d are configured to form one capacitor;electrodes 150 c and 130 e, the other. Overall, by forming these fourcapacitors, device 100 is configured to perform in a fully differentialcapacitive architecture, and device 100 may be referred to as a fullydifferential capacitive MEMS accelerometer. The fully differentialcapacitive architecture allows the differences (e.g., voltage, current,or capacitance) to be measured by another circuit. In some embodiments,a fully differential capacitive architecture may allow the capacitors tobe connected using a full bridge connection or a Wheatstone bridgeconnection. In another embodiment, the fully differential capacitivearchitecture may be connected to differential readout circuitry, forexample, using a differential operational amplifier. In someembodiments, these configurations may avoid the disadvantages of a lowsignal-to-noise ratio found in traditional MEMS accelerometers.

In addition, the architecture shown in FIG. 1 allows measurement ofacceleration along an axis 155 that perpendicularly intersectssubstrates 110, 130, and 150 (referred to as the “Z-axis” herein).Because proof mass 130 a is separated from anchor regions 130 f bycavities 130 g, anchor regions 130 f act as an anchor/stabilizer whenproof mass 130 a moves upwards and downwards along Z-axis 155. Thismovement leads to slight variations in the position of proof mass 130 a,which leads to slight changes in the capacitance of the capacitorsarranged in the fully differential architecture. This change incapacitance allows the capacitors to detect a change in the position ofproof mass 130 a. The fully differential capacitive architecture shownin FIG. 1 thus allows a Z-axis acceleration to be measured.

In another embodiment, device 100 may contain additional electrodes orcapacitors situated surrounding interior substrate 130. With additionalstructural modifications, known to one skilled in the art, theseadditional electrodes or capacitors allow measurement of theacceleration of proof mass 130 a as it moves side-to-side (i.e., to theleft or right of interior substrate 130) or front-to-back (i.e., intoand out of sheet 1). In such an embodiment, device also includes lateralaccelerometer capabilities. Accordingly, in one embodiment, device 100may measure acceleration along Z-axis 155, as well as in an X-Y planeperpendicular to Z-axis 155 (i.e., a plane parallel to substrate 130).This allows an acceleration to be measured or detected in threedimensions.

In one embodiment, the capacitors formed by substrates 110, 130, and 150detect the movement of proof mass 130 a by using a system configured todetect changes in the capacitances. Because sets of electrodes depositedon the substrates are used for sensing the acceleration in device 100,these electrodes may be referred to as sensing electrodes. The systemdetecting the changes in the capacitances may be any system that isconfigured to use the capacitances—for example, a closed-loop readoutcircuit. In other embodiments, along with vacuum packaging andpiezoelectric damping, this capacitive architecture may be used inclosed-loop accelerometer systems, as well as any other resonating MEMSstructure. Together, the four capacitors form a fully differentialarchitecture. In one instance, as proof mass 130 a is displaced alongthe Z-axis by an applied acceleration, two of the capacitors areincreasing in capacitance, while the other two are decreasing equally.The differences in capacitances in each capacitor, as measured by anysystem configured to use capacitances, indicate the position of proofmass 130 a. In certain embodiments, with proper full bridge connectionof these four capacitors, the architecture of device 100 may avoid someof the disadvantages in traditional capacitive sensing architectures—forexample, less sensitivity, low noise suppression, and low SNR. Thesedisadvantages may arise in part from Brownian noise.

The Brownian noise that may be associated with a sensor such as a MEMSaccelerometer may be represented by the following equation:Noise_(MEMS)=√4k _(B) Tb/MIn this equation, k_(B) is Boltzmann's constant (1.381×10⁻²³ J/K), Trepresents the ambient temperature in K, b represents the dampingcoefficient in N/(m/s), and M represents the mass of the resonatingstructure. As can be seen by this equation, thermal noise of the systemcan be decreased by increasing the mass and decreasing the air dampingof the system. By designing a huge mass for the accelerometer, thermalnoise can be decreased down to the order of hundreds of ng/√Hz levels,but practically, MEMS devices are not designed with large sensordimensions.

A high vacuum level may be used to decrease the Brownian noise byreducing the quantity of random interactions of air molecules with thesensor. Accordingly, the use of a vacuum may in some embodiments reducethe noise floor of the system to ng/√Hz levels. Thus in someembodiments, the use of a vacuum-sealed cavity, for example 120, 130 g,and 140, may reduce the Brownian noise inside device 100. In oneembodiment, vacuum-sealed cavities 120, 130 g, and 140 are used toreduce Brownian noise—specifically, the Brownian noise inside device100.

But the use of a vacuum may, in some embodiments, increase the qualityfactor of the system greatly, even over 10,000 levels, which maycontribute to instabilities. To counteract the high vacuum level needed,piezoelectric damping may be used. Piezoelectric damping transforms thekinetic oscillation energy of an accelerometer to electrical energy thatmay be dissipated outside the system, for example, by connecting thepiezoelectric structures to a tunable external load. Thus the qualityfactor may decrease to manageable levels.

Besides the effects of Brownian noise on measurement noise andmeasurement range, non-linearities may affect the performance of MEMSdevices. As one skilled in the art with the benefit of this disclosurewill recognize, non-linearity of a MEMS device may be affected byfrequency response, sensing architecture, springs or the readoutcircuit. These mechanically-related non-linearities may be reduced byusing a closed-loop readout circuit, which may stabilize a proof masswithin a MEMS accelerometer to its original position. In certainembodiments, a closed-loop readout circuit comprises the fullydifferential capacitors, or sensing capacitors, and force feedbackelectrodes. (Force feedback electrodes are discussed more fully belowwith reference to FIG. 4D). With these elements connected in aclosed-loop, the accelerometer may adjust the position of the proof massto maintain linear operation, using the acceleration detected by thecapacitors and a force applied by the force feedback electrodes. Thus,using a closed-loop circuit architecture with a MEMS accelerometer mayavoid some of the disadvantages of non-linearities.

Turning now to FIG. 2, a block diagram illustrating one embodiment of aMEMS accelerometer 200 is shown. As depicted, accelerometer 200 includesupper substrate 210, interior substrate 230, and lower substrate 250. Invarious embodiments, substrates 210, 230, and 250 contain wafers 110 a,130 a and 130 f (regions of the wafer on substrate 230), and 150 arespectively, all of which are similarly numbered to FIG. 1, and may beconfigured as described above with reference to FIG. 1. Additionally, inthe embodiment shown, the wafer of interior substrate 230 is split intothree portions, proof mass 130 a and anchor regions 130 f. In theillustrated embodiment, these portions are separated by cavities 130 gand bounded by protection structures 230 f. In one embodiment,protection structures 230 f may be silicon dioxide. In this embodiment,cavity 120 between upper substrate 210 and interior substrate 230,cavity 140 between lower substrate 250 and interior substrate 230, andcavities 130 g are vacuum-sealed. By vacuum-sealing, orvacuum-packaging, these cavities, certain embodiments of accelerometer200 may avoid some of the disadvantages of Brownian noise discussedabove.

Interior substrates 230 may include several parts: the silicon wafer,composed of proof mass 130 and anchor regions 130 f; cavities 130 g,bounded in part by protection structures 230 f; sets of electrodes 230 band 230 c; spring layers 230 d and 230 e; piezoelectric structures 230j; and pairs of electrodes 230 k situated on piezoelectric structures230 j. In one embodiment, substrates 210 and 250 are divided into fourparts: the wafers of each substrate, 110 a and 150 a respectively; setsof electrodes 210 b and 250 b respectively; oxide layers, 210 c and 250c respectively; and getter layers 210 d and 250 d.

In the embodiment shown, upper substrate 210 is bonded to interiorsubstrate 230, and lower substrate 250 is bonded to interior substrate230 as well. In one embodiment of FIG. 1, bonding between substrates 210and 230 and between 250 and 230 occurs using a precision gap controltechnique. As depicted, substrates 210, 230, and 250 are bonded to eachother using bonding structures 260. Bonding structures 260 may becomposed of any material known to one skilled in the art that maysuitably vacuum seal cavities 120, 130 g, and 140. In one embodiment,bonding structures 260 may be composed of silicon dioxide; in another, ametallic material or composition such as copper and tin. In otherembodiments, bonding structures 260 may be composed of metalliccompositions such as gold and tin, or aluminum and germanium.Alternately, bonding structures 260 may be composed of both silicondioxide and metallic contacts. Cavities 120 and 140 may be vacuum-sealedin part by bonding structures 260. Substrates 210, 230, and 250 may alsoassist in vacuum-sealing cavities 120 and 140. In some embodiments,cavities 120 and 140 may be vacuum-sealed in part by bonding substrates210, 230, and 250 together. Spring layers 230 d and 230 e may haveportions etched away such that vacuum-sealed cavities 120, 130 g, and140 may be in fluid communication with each other (e.g., they maypossess a common vacuum). Thus, the vacuum-sealed cavity, comprisingcavities 120, 130 g, and 140, may be bounded in part by upper substrate210, lower substrate 250, and protection structures 230 f. Bondingstructures 260 and substrate 230 may also bound in part the commonvacuum throughout cavities 120, 130 g, and 140. Vacuum-sealed cavities120, 130 g, and 140 may assist in avoiding noise (e.g., Brownian noise)caused by the movement of proof mass 130 a.

In one embodiment, spring layers 230 d and 230 e are grown/deposited onopposing surfaces of interior substrate 230. As used herein, the term“grown” refers to any fabrication technique in which a type of materialis placed on at least a portion of an underlying material or layer byheating that material or layer to high temperatures. For example,heating a silicon substrate to high temperatures may create bonds withoxygen atoms in the air so that silicon dioxide is formed. Thus anothermaterial or layer may be grown by this thermal oxide growth. Springlayers 230 d and 230 e may be composed of an oxide such as silicondioxide. Spring layers 230 d and 230 e allow proof mass 130 a to vary inposition within interior substrate 230, with anchor regions 130 fassisting by adding stability to interior substrate 230. Oxide layers210 c and 250 c are grown, or disposed, on the lower surface of uppersubstrate 210 and the upper surface of lower substrate 250 respectively.Oxide layers 210 c and 250 c may be composed of silicon dioxide. Getterlayers 210 d and 250 d, which assist in maintaining the common vacuum ofvacuum-sealed cavities 120, 130 g, and 140, are deposited on oxidelayers 210 c and 250 c. In some embodiments, getter layers 210 d and 250d may be deposited on any portion of substrates 210, 230, and 250exposed to the vacuum-sealed cavity. In one embodiment, a single getterlayer may exist within accelerometer 200, deposited on some portion ofsubstrates 210, 230, and/or 250. Getter layers 210 d may be composed ofany suitable material known to those skilled in the art, and may assist,in some embodiments, in avoiding some of the disadvantages of Browniannoise within vacuum-sealed cavities 120, 130 g, and 140.

As shown, two sets of electrodes 230 b and 230 c may be deposited onspring layers 230 d and 230 e—the first on the upper surface of interiorsubstrate 230; and the second, on the lower surface. Said differently,sets of electrodes 230 b and 230 c may be deposited on opposite sides ofthe interior substrate. A set of electrodes 210 b is deposited on thelower surface of upper substrate 210. A set of electrodes 250 b is alsodeposited on the upper surface of lower substrate 250. Both sets ofelectrodes 210 b and 250 b on upper substrate 210 and lower substrate250 respectively may be referred to as a set of electrodes deposited onan opposing surface from the upper and lower surface respectively ofinterior substrate 230.

In the embodiment shown, sets of electrodes 210 b and 230 b areconfigured to form two capacitors. Similarly, sets of electrodes 230 cand 250 b are configured to form two capacitors. Overall, by formingthese four capacitors, accelerometer 200 is configured to perform in afully differential capacitive architecture, for example, as describedabove with reference to FIG. 1. Accordingly, in the embodiment depictedin FIG. 2, the fully differential capacitive architecture may allow thecapacitors to operate together to detect changes in an acceleration ofproof mass 130 a as it moves upwards and downwards along Z-axis 155. Forexample, the capacitors formed by sets of electrodes 210 b, 230 b, 230c, and 250 b may detect a change in the acceleration of accelerometer200. Then, a closed-loop circuit or system may determine an accelerationof accelerometer using the measured electrical current, change incapacitance, or change in voltage of these capacitors. In someembodiments, this closed-loop circuit or system may be referred to asfront-end readout circuitry, which may use a differential operationalamplifier configuration. In some embodiments, accelerometer 200 maycontain additional electrodes or capacitors situated surroundinginterior substrate 230. With additional structural modifications knownto one skilled in the art these additional electrodes or capacitorsallow measurement of acceleration in an X-Y plane perpendicular toZ-axis 155. Such modifications would allow acceleration to be measuredor detected in three dimensions.

In one embodiment, accelerometer 200 also includes piezoelectricstructures 230 j disposed on spring layers 230 d and 230 e.Piezoelectric structures 230 j may be composed of any piezoelectricmaterial. Piezoelectric structures 230 j translate mechanical energyfrom spring layers 230 d and 230 e into electrical energy, which may bemeasured by pairs of electrodes 230 k disposed on each piezoelectricstructure 230 j. Further, this electrical energy may be dissipatedexternally to decrease the overall energy of the system. Thepiezoelectric material may bend due to the movement of proof mass 130 a,which is translated to mechanical energy by spring layers 230 d and 230e. The addition of this piezoelectric damping, together in operationwith sets of electrodes 210 b, 230 b, 230 c, and 250 b forming a fullydifferential capacitive architecture, may reduce the Q-factor ofaccelerometer 200 in a closed-loop system. The Q-factor may be adjustedby various readout circuitries based on the measurements frompiezoelectric structures 230 j.

FIGS. 3A-C illustrate an exemplary process flow for the fabrication of acap substrate. Turning now to FIG. 3A, substrate 310 may be a siliconwafer, etched for the later deposition of getter layers. Layer 320 isdeposited or grown on substrate 310. Layer 320 may be further patterned.In one embodiment, layer 320 may be silicon dioxide. Turning now to FIG.3B, a set of electrodes 350 and metallic contacts 360 and 365 aredeposited on layer 320. Notably, the set of electrodes 350 are isolatedfrom one another on layer 320. Set of electrodes 350 may be any type ofmetallic contact. Metallic contacts 360 and 365 may be chromium, whichmay be patterned with lift off. In this embodiment, layer 320 ispatterned further for the deposition of metallic contacts 365. In otherembodiments, metallic contacts 360 and 365 may be deposited on anotherlayer, which may be silicon dioxide, especially patterned for theirdeposition. This additional layer may be deposited partially on layer320, for example, deposited only in the regions of metallic contacts 360and 365.

Turning now to FIG. 3C, spacers 370 are deposited on layer 320. In someembodiments, spacers 370 may also be deposited on another layer, whichmay be silicon dioxide, especially patterned for their deposition. Asdepicted, spacers 370 may be silicon dioxide. In some embodiments,metallic contacts 360 and 365 and spacers 370 may operate as a bondingregion to be bonded to another substrate as described below withreference to FIG. 4. In one embodiment, spacers 370 may be referred toas bonding spacers.

FIGS. 4A-F illustrate an exemplary process flow for the fabrication of afully differential MEMS accelerometer. Turning now to FIG. 4A, substrate410 may be a silicon wafer. Trenches 415 may be filled with silicondioxide. In one specific embodiment, trenches 415 may be 3 μm wide. Tofill trenches 415, trenches 415 may be etched first by any method knownto one skilled in the art. For example, in one embodiment, using deepreactive-ion etching (DRIE), 3 μm wide trenches are opened on thesilicon wafer. Then, to fill trenches 415, oxide is grown on the surfaceof substrate 410. In another embodiment, this oxide may be used as amasking layer for etching in later fabrication stages, for example, XeF₂(gaseous) etching to remove portions of substrate 410. The depth oftrenches 415 may affect the thickness of the accelerometer mass becausesubstrate 410 is part of the fully fabricated accelerometer. Referringbriefly to FIG. 4D, because trenches 415 isolate proof mass 410 a fromanchor regions 410 f, trenches 415 may be referred to as isolationtrenches. Trenches 415 also protect proof mass 410 a and anchor regions410 f from possible later etching steps. Thus trenches 415 may also bereferred to as protection trenches. Layer 420 is deposited/grown onsubstrate 410, also covering trenches 415. Layer 420 may be silicondioxide. In one embodiment, layer 420 may also be patterned fordeposition of subsequent layers or deposited portions. In thisaccelerometer embodiment, layer 420 may be referred to as a “springlayer.” In one specific embodiment, the thickness of layer 420 may be 4μm.

Turning now to FIG. 4B, metallic contacts 425 are deposited andpatterned for deposition of piezoelectric structures 430 (also referredto as piezoelectric layers). Metallic contacts 425 (also referred to asbottom electrodes) may be various metals, known to one skilled in theart. Piezoelectric structures 430 may be various piezoelectricmaterials, known to one skilled in the art. To form piezoelectricstructures 430, piezoelectric material is deposited. In certainembodiments, both metallic contact 425 and piezoelectric structure 430may be referred to as the piezoelectric structure.

Turning now to FIG. 4C, layer 440 is deposited/grown on layer 420 andpatterned to protect the side walls of piezoelectric structures 430.Layer 440 (also referred to as sidewall protection) may be silicondioxide. Then, top metallization is deposited on layer 440. Thismetallic deposition forms set of electrodes 450 (also referred to ascontact electrodes). Set of electrodes 450 are deposited so that theelectrodes are isolated from each other on layer 440. In one embodimentwith a further etching step, set of electrodes 450 may also bepatterned. Layer 440 may also include another thin layer of silicondioxide. That layer may be patterned so that the bonding regions, theregions extending laterally outwards from piezoelectric structures 430(or the region surrounding and including bonding pads 427) are definedfor the later bonding of spacers 460, which are depicted in FIG. 4D.These bonding regions may also be referred to as wafer bonding areas.Bonding pads 427 are deposited in the same metallic deposition as setsof electrodes 450. In one embodiment, bonding pads 427 may also bepatterned, for example, especially for later bonding of a cap wafer.Finally, in the same metallic deposition, pairs of electrodes 455 aredeposited on piezoelectric structures 430 and partially on layer 440. Inthe wafer bonding areas, set of electrodes 450, and pairs of electrodes455, chromium may be patterned with lift off. In another embodiment,bonding pads 427, sets of electrodes 450, and pairs of electrodes 455may be deposited and patterned in separate steps. For example, theseelements may be deposited or electroplated. In some embodiments, themetals used for bonding pads 427, sets of electrodes 450, and pairs ofelectrodes 455 may be gold, aluminum, or chromium.

Turning now to FIG. 4D, cap wafer 475 is bonded to substrate 410 by anybonding process known to one skilled in the art. As part of substrate410, proof mass 410 a is bounded in part by trenches 415 and springlayers 420 and 480. In one embodiment, cap wafer 475 may be bonded tosubstrate 410 by any suitable method to known to one skilled in the art.For example, various bonding methods may be used to align spacers 460with the bonding region on substrate 410 and bonding pads 427 with anopposing contact on cap wafer 475 between bonding spacers 460. This mayalso assist in determining spacing between substrate 410 and cap wafer475. In some embodiments, for example in accelerometer 200 as describedabove with reference to FIG. 2, such a bonding method may determine theheight of cavities 120 and 140. Because of this bonding process, in oneembodiment, spacers 460 may be referred to as bonding spacers. In someembodiments, cap wafer 475 may be a cap wafer fabricated by the processillustrated in FIG. 3. Thus cap wafer 475 contains layer 470, which maybe silicon dioxide, isolating the set of electrodes on cap wafer 475opposing set of electrodes 450.

During this bonding process, set of electrodes 450 are aligned to opposethe set of electrodes on cap wafer 475 so that at least a portion ofthese sets of electrodes may form the capacitors to be used in a fullydifferential capacitive architecture. In some embodiments, variousbonding methods known to one skilled in the art may assist indetermining the spacing of set of electrodes 450 from the set ofelectrodes on cap wafer 475. In certain embodiments, the centerelectrode of set of electrodes 450 and the opposing electrode on capwafer 475 may form electrode contacts to be used for force feedback.That is, these electrodes are operable to apply a force to proof mass410 a.

MEMS accelerometers, to operate in a regime of approximate linearity,may use electrodes to apply a force to the proof mass. In the embodimentdepicted in FIG. 4D, the center electrode of set of electrodes 450 andthe opposing electrode on cap wafer 475 may form electrode contacts tobe used for force feedback. In some embodiments, this may avoid some ofthe disadvantages of MEMS accelerometers that use electrodes for sensingand force feedback at the same time. Certain MEMS accelerometers mustswitch between integration and feedback in a closed loop circuit, whichmay increase circuit complexity and may decrease the maximum averagefeedback force applied. But to operate in a closed loop circuit,accelerometers may need to apply force to the proof mass or structure.Thus in the embodiment shown, separate electrodes (namely, the centerelectrode of set of electrodes 450 and the opposing electrode on capwafer 475) are used to apply force to the proof mass. Because theseelectrodes are used solely to apply force, these electrodes may bereferred to as force feedback electrodes. These electrodes receivefeedback from an external circuit to apply a force to proof mass region,which may allow accelerometer 400 to avoid operating in a non-linearmanner. Such force feedback electrodes may also allow accelerometer 400to avoid switching complexity from an external circuit and may increasethe measurement range of accelerometer 400.

After cap wafer 475 is bonded to substrate 410, substrate 410 is groundfrom bottom up to the tip of trenches 415. In some embodiments,substrate 410 may be ground somewhat beyond the tips of trenches 415.Then layer 480 is grown/deposited on the bottom (or may be referred toas backside) of substrate 410. In one specific embodiment, the thicknessof layer 480 may be 4 μm. In this accelerometer embodiment, layer 480 isreferred to as a spring layer.

Turning now to FIG. 4E, layer 481 (also referred to as sidewallprotection), piezoelectric structures 486 (also referred to aspiezoelectric layers) including metallic contacts/bottom electrodes, setof electrodes 490, pairs of electrodes 493 (also referred to as contactelectrodes), and bonding pads 494 are deposited on to layer 480 usingthe same or similar process outlined above with reference to FIGS. 4A-C,with similar corresponding elements. Substrate 410 may be etched to formcavities between trenches 415 as described below with reference to FIG.5. After etching, proof mass 410 a is separated from anchor regions 410f by trenches 415 and the cavities bounded in part by trenches 415.

Turning now to FIG. 4F, cap wafer 495 is bonded to substrate 410 usingthe same or similar process described above with reference to FIG. 4D.Various bonding methods known to one skilled in the art may alignspacers 460 with the bonding region on substrate 410 and align bondingpads with an opposing contact on cap wafer 495 between bonding spacers460. Such bonding methods may assist in determining spacing betweensubstrate 410 and cap wafer 495. In some embodiments, cap wafer 495 maybe a cap wafer fabricated by the same or similar process illustrated inFIG. 3. Thus cap wafer 495 contains layer 470, which may be silicondioxide, isolating the set of electrodes on cap wafer 495 opposing thecorresponding set of electrodes on substrate 410, at least a portion ofthese sets of electrodes may form the capacitors to be used in a fullydifferential capacitive architecture. Thus substrate 410 and cap wafers475 and 495 are now fabricated to form a fully differential MEMSaccelerometer. In certain embodiments, the bottom center electrode ofsubstrate 410 and center electrode on cap wafer 495 may form electrodecontacts to be used for force feedback. In some embodiments, for examplein accelerometer 200, this last bonding step may form a commonvacuum-sealed cavity throughout cavities 120, 130 g, and 140.

FIGS. 5A-E illustrate an exemplary process flow for the etching ofcavities within a substrate. Turning now to FIG. 5A, substrate 510 maybe a silicon wafer. Trenches 515 may be etched by any method known toone skilled in the art. For example, in one embodiment, using deepreactive-ion etching (DRIE), 3 μm wide trenches are opened on thesilicon wafer. Trenches 515 may be used as protection layers, orprotection structures, during the later isotropic release processes. Thedepth of trenches 515 may affect the thickness of the accelerometermass. In an accelerometer implementation, for example in accelerometer200, trenches 515 may be referred to as protection trenches.

Turning now to FIG. 5B, trenches 515 are grown/filled/deposited, with anoxide, for example silicon dioxide conformally. This filling processgrows a layer of oxide on the surface of 510, which is removed with CMP.Then layer 520 is grown/deposited on substrate 510. The thickness oflayer 520, which may be silicon dioxide, may affect the thickness of thespring layers used in an accelerometer implementation. For example,layer 520 may be spring layer 230 d on substrate 230 in accelerometer200. Thus, in some embodiments, precise thickness control of layer 520may be used during deposition.

Turning now to FIG. 5C, a bottom portion of substrate 510 is removed,for example through grinding and (chemical mechanical polishing) CMP.The removal may be up to the bottom of trenches 515. Then, turning nowto FIG. 5D, layer 540 is grown/deposited on substrate 510. The thicknessof layer 540, which may be silicon dioxide, may affect the thickness ofa spring layer used in an accelerometer implementation. For example,layer 520 may be spring layer 230 e on substrate 230 in accelerometer200. Thus, in some embodiments, precise thickness control of layer 540may be used during deposition. Layers 520 and 540 may be surfacepatterned for use as spring layers in an accelerometer embodiment.

Turning now to FIG. 5E, using both photo resist and silicon dioxide asmask layer, bulk silicon regions between trenches 515 are etched throughsubstrate 510. In some embodiments, this etching process may beperformed using dry vertical etching techniques, known to one skilled inthe art. In some embodiments, the etching may be omitted; it may beadvantageous to conduct the etching, however, in order to decrease theprocessing time of the subsequent processing step. After these trenchesare etched bonded wafers are placed into XeF₂ (gaseous) for isotropicrelease of substrate 510. Silicon dioxide covering all surfaces (i.e.,through layers 520 and 540 and filled trenches 515) of substrate 510 actas a masking layer. Substrate 510 is etched as shown in FIG. 5E, leavingcavities 550. By vacuum-sealing, or vacuum-packaging, cavities 550,implemented in an accelerometer, may avoid some of the disadvantages ofBrownian noise discussed above.

Overall, FIGS. 5A-E depict one embodiment of a method comprising:etching at least two trenches in a substrate; depositing a first supportlayer on the upper surface of the substrate; depositing a second supportlayer on the lower surface of the substrate; etching the substratebounded by the trenches and the first and second support layers. Themethod may further comprise, wherein etching the trenches in thesubstrate includes: filling the trenches with a support material;removing a portion of the support material; and removing a portion ofthe lower surface of the substrate. The method may further comprise,wherein depositing the first and second support layer further includes:patterning the first and second support layers. The method may furthercomprise, wherein the etching the substrate further includes: etchingthe interior of the substrate to form at least one vacuum-sealed cavitybounded by the trenches and the first and second support layers.

Turning now to FIG. 6, a method in accordance with one embodiment ofthis disclosure is provided. Flow begins at step 600.

At step 600, a survey vessel tows a streamer including at least oneaccelerometer in accordance with this disclosure. In variousembodiments, the streamer may include a plurality of accelerometers inaccordance with this disclosure, and it may also include other sensors(e.g., pressure sensors and/or electromagnetic sensors). In someinstances, the survey vessel may tow a plurality of such streamers. Flowproceeds to step 602.

At step 602, one or more seismic sources are actuated. These may belocated on the survey vessel, towed by the survey vessel, towed by adifferent vessel, etc. Seismic energy from the seismic sources travelsthrough the water and into the seafloor. The seismic energy thenreflects off of the various geophysical formations. Various portions ofthe seismic energy may then be reflected upward toward the streamer, insome instances incorporating time delays and/or phase shifts that may beindicative of the geophysical formations. Flow proceeds to step 604.

At step 604, seismic energy is received at the accelerometers located onthe streamers. Different portions of the seismic energy may reach theaccelerometers either directly from the seismic sources, or after one ormore reflections at the seafloor and/or water surface. Data based on thereceived seismic energy may then be used to infer information aboutgeological structures that may exist under the seafloor. Flow ends atstep 604.

Turning now to FIG. 7, an additional method in accordance with oneembodiment of this disclosure is provided. Flow begins at step 700.

At step 700, a survey vessel tows streamers including acoustictransmitters, and also including accelerometers in accordance with thisdisclosure. In some instances, the acoustic transmitters and theaccelerometers may be combined into an acoustic transceiver. Flowproceeds to step 702.

At step 702, one or more of the acoustic transmitters are actuated. Theacoustic energy produced by the transmitters may travel through thewater toward the other streamers. Flow proceeds to step 704.

At step 704, the acoustic energy is received by an accelerometer. Thedelay between the actuation of the acoustic transmitters and thereception at the accelerometer may be based in part on the distancebetween them. Flow proceeds to step 706.

At step 706, the positions of the streamers (or portions thereof) aredetermined. For example, such positions may be determined based on thedistances between pairs of acoustic transmitters and accelerometers.Flow ends at step 706.

One of ordinary skill in the art with the benefit of this disclosurewill understand that various aspects of this disclosure may in someembodiments be implemented via computer systems. Such computer systemsmay in some embodiments include various types of non-transitorycomputer-readable media, such as hard disks, CDs, DVDs, RAM, ROM, tapedrives, floppy drives, etc.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. A method, comprising: towing a streamer behind asurvey vessel in a body of water, wherein the streamer includes anaccelerometer; detecting, by at least four capacitors within theaccelerometer, a change in acceleration of the accelerometer, wherein:the four capacitors include a first capacitor and a second capacitor ona first side of a proof mass and a third capacitor and a fourthcapacitor on a second side of the proof mass, and each of the fourcapacitors includes a respective pair of electrodes; and in response tothe proof mass moving in a selected direction, a capacitance of thefirst and second capacitors is operable to increase, and a capacitanceof the third and fourth capacitors is operable to decrease; anddetermining an acceleration of the accelerometer based at least in parton the detecting.
 2. The method of claim 1, wherein detecting the changein acceleration includes measuring an electrical current across the atleast four capacitors.
 3. The method of claim 2, wherein measuring theelectrical current includes: measuring a change in capacitance of the atleast four capacitors; and measuring a change in voltage of the at leastfour capacitors.
 4. The method of claim 1, wherein determining theacceleration includes determining the Z-axis acceleration.
 5. The methodof claim 1, wherein determining the acceleration includes usingfront-end readout circuitry connected to the at least four capacitors.6. The method of claim 1, further comprising: actuating a seismic energysource to produce seismic energy, wherein the acceleration of theaccelerometer is based at least in part on the seismic energy.
 7. Themethod of claim 1, further comprising: towing another streamer behindthe survey vessel in the body of water; and actuating an acoustictransmitter on the another streamer to produce acoustic energy, whereinthe acceleration of the accelerometer is based at least in part on theacoustic energy.
 8. A sensor configured to receive seismic energy, thesensor comprising: an accelerometer that includes: a first substrateincluding a proof mass; first and second spring layers respectivelydisposed on a first surface and a second, opposite surface of the firstsubstrate; first and second sets of at least two electrodes respectivelydisposed on the first and second spring layers; a second substratespaced from the first spring layer, wherein a third set of at least twoelectrodes are disposed on the second substrate at locationscorresponding to those of the first set of electrodes; and a thirdsubstrate spaced from the second spring layer, wherein a fourth set ofat least two electrodes are disposed on the third substrate at locationscorresponding to those of the second set of electrodes.
 9. The sensor ofclaim 8, wherein the first and third sets of electrodes and the secondand fourth sets of electrodes respectively form capacitors operable todetect variations in the proof mass.
 10. The sensor of claim 8, whereinelectrodes in the first, second, third, and fourth sets of electrodesare operable at least in part to apply a force to the proof mass. 11.The sensor of claim 8, wherein the second substrate is spaced from thefirst spring layer by a vacuum-sealed cavity, wherein the thirdsubstrate is spaced from the second spring layer by the vacuum-sealedcavity.
 12. The sensor of claim 11, wherein the vacuum-sealed cavity isbounded in part by bonding structures including metallic and silicondioxide portions.
 13. The sensor of claim 8, wherein each electrode inthe first and second set of electrodes includes: an oxide portiondisposed on the first spring layer; and a metal contact disposed on theoxide portion.
 14. The sensor of claim 13, wherein the first substratefurther includes first and second sets of piezoelectric structuresrespectively disposed on the first and second spring layers, whereineach piezoelectric structure in the first and second sets ofpiezoelectric structures includes: a metallic portion disposed on thefirst spring layer; a piezoelectric contact disposed on the metallicportion; first and second oxide portions respectively disposed onopposite sides of the piezoelectric contact; and a pair of electrodesdisposed on the first and second oxide portions.
 15. A sensor configuredto receive seismic energy, the sensor comprising: an accelerometer thatincludes: a central substrate region; a first bonded substrate opposinga first surface of the central substrate region; a second bondedsubstrate opposing a second surface of the central substrate region; afirst pair of capacitors formed between the first bonded substrate andthe central substrate region; and a second pair of capacitors formedbetween the second bonded substrate and the central substrate region,wherein each of the first pair and second pair of capacitors includes arespective pair of electrodes, and wherein in response to anacceleration in a selected direction, a capacitance of the first pair ofcapacitors is operable to increase, and a capacitance of the second pairof capacitors is operable to decrease.
 16. The sensor of claim 15,wherein the central substrate region includes: a proof mass regionbounded by a first spring structure, a second spring structure, a firstprotection structure, and a second protection structure.
 17. The sensorof claim 16, further comprising: a vacuum-sealed cavity bounded in partby the first and second bonded substrates, the first and secondprotection structures, a third protection structure, and a fourthprotection structure.
 18. The sensor of claim 17, wherein the first,second, third, and fourth protection structures are disposed laterallyon either side of the proof mass region, and wherein the first andsecond bonded substrates are disposed vertically on either side of thecentral substrate region.
 19. The sensor of claim 17, wherein the first,second, third, and fourth protection structure include silicon dioxide.20. The sensor of claim 15, wherein the first bonded substrate includesa first getter layer, wherein the second bonded substrate includes asecond getter layer.
 21. A sensor configured to receive seismic energy,the sensor comprising: an accelerometer that is a fully differentialMEMS accelerometer configured to measure Z-axis acceleration of a proofmass, wherein the accelerometer includes: a proof mass; a firstcapacitor and a second capacitor on a first side of the proof mass; anda third capacitor and a fourth capacitor on a second side of the proofmass; wherein each of the four capacitors includes a respective pair ofelectrodes; and wherein in response to the proof mass moving in aselected direction, a capacitance of the first and second capacitors isoperable to increase, and a capacitance of the third and fourthcapacitors is operable to decrease.
 22. The sensor of claim 21, whereinthe sensor includes a central substrate region including the proof mass,two anchor regions, and two portions of a vacuum-sealed cavity.